Photographic techniques using a silver halide have heretofore been most widely utilized, since the photographic characteristics thereof, such as sensitivity, gradation control, etc., are superior to those of other photographic techniques such as electrophotography or diazo photography. Recently, an improved photographic technique has been developed capable of simply and rapidly forming an image, where the image-formation of a silver halide-containing photographic material is carried out by means of a dry process using heat instead of a conventional wet processing using a developing liquid or the like.
A heat-developable photographic material is known in this technical field, and various heat-developable photographic material and processes thereof are described, for example, in Bases of Photographic Industry, Corona Publishing, 1979, pp. 553-555; Film Information (April, 1978), p. 40; Neblette's Handbook of Photography and Reprography, 7th Ed., Van Nortrand Reinhold Company, 1977, pp. 32-33; U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075; British Pat. Nos. 1,131,108 and 1,167,777; and Research Disclosure, RD No. 17029 (June, 1978), pp. 9-15.
Many processes have been proposed for formation of color images. As for means for formation of color images due to binding of a developing agent, as oxidized, and a coupler, U.S. Pat. No. 3,531,286 describes a combination of a p-phenylene-diamine reducing agent and a phenolic or active methylene-coupler; U.S. Pat. No. 3,761,270 described a p-aminophenol reducing agent; Bergian Pat. No. 802,519 and Research Disclosure, RD No. 13742 (September, 1975), pp. 31-32 describe a sulfonamidophenol reducing agent; and U.S. Pat. No. 4,021,240 describes a combination of a sulfonamido-phenol reducing agent and a four-equivalent coupler.
Other methods for formation of positive color images include a photographic silver dye-bleaching process, as described, for example, in Research Disclosure, RD No. 14433 (April, 1976), pp. 30-32 and RD No. 15227 (December, 1976), pp. 14-15, and U.S. Pat. No. 4,235,957, which disclose useful dyes and bleaching processes.
In addition, European Patent Publication (unexamined) Nos. 76,492 and 79,056 and Japanese Patent Application (OPI) Nos. 28928/83 and 26008/83 disclose a method for formation of color images by heat-development, using a compound essentially having a dye moiety and capable of releasing a mobile dye corresponding or reversely corresponding to the reduction reaction to reduce a silver halide to silver at a high temperature. The term "OPI" indicates an unexamined patent application open to public inspection.
In said image-forming methods, an alkaline agent or an alkali precursor is generally incorporated in a photographic material, in order to accelerate the development of said material under heat. However, such photographic materials as comprising a combination of a silver halide emulsion which has been color-sensitized with a sensitizing dye and an alkaline agent or an alkali precursor have a serious defect in that the sensitivity of the material is usually lowered during the preservation thereof.
In particular, in the case that a color-sensitized silver halide is used in a photographic system containing said dye and a compound capable of releasing a mobile dye corresponding to or reversely corresponding to the reduction reaction or reducing said silver halide to silver at a high temperature, the preservation stability of the material is especially remarkably lowered. The reason is thought to be as follows: As the dye-releasing compound itself has a dye moiety, it thereby has characteristics of a dye, and so, if said dye-releasing compound is co-used together with a silver halide which has been color-sensitized with a sensitizing dye, said dye-releasing compound may react with said sensitizing dye which has adsorbed to the silver halide, resulting in desorption of said sensitizing dye from the surface of the silver halide during preservation (before use) of the photographic material. Such defect can be fatal in color photographic materials or other photographic materials in the case of the electromagnetic radiation falling outside of the range of the intrinsic sensitivity of a silver halide.